1. Field of the Invention
The present invention relates to a photopolymer elastomeric article which is designed to be used in a flexographic printing system. Flexographic printing is a method of rotary letterpress printing which employs flexible elastomer printing plates. In this system the ink is transferred directly from the raised surfaces of the elastomeric plate to the printing stock. Those properties most desirable in flexible photopolymer plates include resistance to the wide variety of solvents used in printing inks, good ink transfer from plate to substrate, proper hardness (durometer), short plate processing time, resistance to ozone and oxidative cracking, and resistance to post-hardening.
Flexible photopolymer printing plates in the prior art have usually been made up of:
1. An elastomeric polymer matrix;
2. A photopolymerizable, ethylenically unsaturated crosslinking agent, as typified by esters of acrylic or methacrylic acid ("monomer"); and
3. A photoinitiator, which is a compound, which when irradiated, will initiate polymerization and/or crosslinking.
The chemical nature of these three elements has been varied and additional components have been added as needed to improve plate production and performance characteristics, especially printing quality.
It has been assumed in the prior art that large amounts, i.e., greater than 5 phr (parts per hundred of rubber) of the photopolymerizable ethylenically unsaturated, crosslinking agent are required to produce a commercially acceptable photopolymer flexographic printing plate. The current invention, however, employs a mixture of syndiotactic 1,2-polybutadiene (SBD) and 30% by weight or greater of cis-1,4-polyisoprene (cis-IR), which requires no, or optionally no more than 3 phr of crosslinking agent. The resultant flexographic printing plate shows excellent printing qualities, is highly resistant to the ozone cracking caused by the high concentrations of ozone in the areas of some flexographic presses, and is easy to process.
2. Description of the Prior Art
U.S. Pat. No. 2,760,863 discloses a process to make relief images from a photopolymerizable system containing a polymeric thickener or filler, an addition polymerizable monomer, and a photopolymerization initiator.
U.S. Pat. No. 3,024,180 and Canadian No. 614,181 disclose relief plates based on polymers of 1,3-butadiene and halogen-substituted 1,3-butadiene, and mention copolymers thereof with aryl olefins, e.g., styrene, acrylic esters and nitriles.
U.S. Pat. No. 3,674,486 discloses printing plate compositions based on block copolymers of styrene and butadiene (SBS) or isoprene (SIS).
U.S. Pat. No. 3,764,324 discloses a printing plate formulation consisting of an unsaturated copolymer derived from butadiene and acrylonitrile, and a saturated halogenated polymer, such as polyvinylchloride, which results in a hard solvent resistant plate.
British Pat. No. 1,552,653 discloses the use of SBD as a polymer matrix and of cis-IR as a plasticizer therewith, but teaches the use of a monomer in much greater amounts than 3 phr (at least 5 phr) as being essential to produce an acceptable product, and discloses only up to 23% cis-IR in said mixture.
U.S. Pat. No. 4,103,057 and its British equivalent British Pat. No. 1,481,212, which have been assigned to the same assignee as British Pat. No. 1,552,653, disclose a photocurable SBD composition with no monomer used for insolubilizing previously mechanically molded patterns, but do not teach the use of those compositions to produce such patterns by photochemical insolubilization.
U.S. Pat. No. 4,045,231 discloses a photosensitive resin composition for flexographic printing plates comprising (a) a styrene-butadiene block copolymer containing 35-50% by weight of styrene; (b) at least one liquid prepolymer having a molecular weight of 1,000 to 5,000 selected from the group consisting of polybutadiene and butadiene-styrene copolymer; (c) a monomer containing at least one vinyl group, such as glycol methacrylates; and (d) a photopolymerization initiator, such as benzophenone.
U.S. Pat. No. 4,137,081 describes photopolymer compositions containing a liquid polymer having at least two terminal olefin groups attached to the polymer and photopolymerized in the presence of from 1 to about 50% by weight, based on the liquid polymer, of at least one ethylenically unsaturated monomer.
None of the above references disclose a flexible photopolymer plate in which the polymer matrix is a mixture of SBD and cis-IR, with cis-IR being 30% or greater, with no or optionally, no more than 3 phr of photopolymerizable crosslinking agent.